1. Field of the Invention
This invention relates to electronic article surveillance systems, and more particularly to a transceiver antenna having a core made of an amorphous magnetic material for electronic article surveillance marker detection.
2. Description of the Related Art
Electronic article surveillance (EAS) systems are typically used to protect assets including reducing theft of retail articles. In operation, an EAS interrogation zone is established around the perimeter of a protected area such as the exits of a retail store. EAS markers, which are detectable within the interrogation zone, are attached to each asset or article to be protected. The interrogation zone is established by EAS antennas positioned for example, in the vicinity of the store's exit. The EAS antennas transmit an electromagnetic interrogation field, which causes a response from an active EAS marker in the interrogation zone. The EAS antennas receive and the EAS electronics detect the EAS marker's response, which indicates an article, with an attached EAS marker, is in the interrogation zone. EAS markers are removed, or the markers deactivated, for articles purchased or otherwise authorized for removal from the store or protected area. Hence, an EAS marker detected within the interrogation zone indicates that an article is attempting to be removed from the protected area, or store, without authorization, and appropriate action can be taken.
The EAS antennas, which are typically made of air core coils of wire, may be configured as separate transmit and receive antennas, or as transceiver antennas. These conventional EAS air-core antennas must generate interrogation zones that are sufficient to cover stores that have very wide exits, and are relatively large. In food and other stores, having narrow aisles the smallest antennas possible are desired. In these narrow aisle environments EAS antennas must operate near metal surfaces and check-stands, which can result in degraded performance. Expensive, large, and heavy shielding is required for conventional air-core EAS antennas for effective operation in this environment. There exists a need for smaller EAS antennas that perform satisfactorily, especially in tight spaces and near metal surfaces.
The use of ferrite core EAS receive antennas is well known. Ferrite material is a powder, which is blended, compressed into a particular shape, and then sintered in a very high temperature oven. It is a compound that becomes a fully crystalline structure after sintering. Ferrite has a higher magnetic permeability than air effectively increasing the detection performance of a ferrite core antenna. A ferrite core receiver antenna sold by Sensormatic uses a manganese zinc ferrite rod about 19 cm long and 0.6 cm in diameter with magnet wire wound about the surface. However, in certain EAS frequency bands of interest and at required levels of excitation field, ferrite cores may saturate before producing an interrogation field suitable for detecting EAS markers at a useable distance.
The use of amorphous magnetic material core antennas is known for certain receiver applications. U.S. Pat. No. 5,220,339, to Matsushita, discloses a receiver antenna having an amorphous core for UHF and VHF television frequency reception. The '339 patent discloses two magnetic core geometries. The first core geometry is a solid cylindrical shape made of amorphous fibers. The second core geometry is a hollow cylindrical shape made of an amorphous sheet spiral rolled to form a hollow cylinder. A conductive insulated winding surrounds each core. The magnetic permeability of amorphous metal is significantly higher than ferrite, indicating improved reception performance in comparison to a ferrite core at certain frequencies. The '339 patent provides no useable information or teaching directed toward transmitting using an amorphous core antenna.
U.S. Pat. No. 5,567,537, to Yoshizawa et al., discloses a passive transponder antenna using a magnetic core for identification systems applications. A remote transmitter field source produces an induced voltage on the transponder antenna that energizes the transponder transmitting/receiving device, which then transmits a digital code to a remote receiver antenna. The transponder core antenna uses a very thin magnetic core and is not directly coupled to the electronics that powers the remote transmitter and receiver antennas. The magnetic core element, which can be an amorphous alloy, is 25 microns thick or less. A thickness greater than 25 microns is not suitable due to decreased Q and lower sensitivity. The lower the thickness, the better the performance, and, as stated in the '537 patent at column 5, lines 1-6, 15 microns thickness is better than 25 microns. The thickness of the laminated core antenna, which is made up of a plurality of core elements, is disclosed to be 3 mm or less. The target frequency for the identification system is 134 kHz. The preferred Q value is greater than 25 or 35, or even more, at the 134 kHz frequency. The power levels operating the passive transponder are quite low, and the level of magnetic field transmitted by such a device is extremely low.